Sound perception is mediated through a specialized sensory epithelium made up of mechanosensory hair cells and supporting cells. The proper development and maintenance of this epithelium is critical for hearing, and loss of these cells leads to deafness. While many vertebrates, including birds and reptiles, are able to regenerate damaged hair cells, mammals (including humans) do not retain this ability, resulting in permanent deafness after injury. Gene therapy is a potentially promising therapeutic intervention that may one day allow for the recovery of hearing in deaf individuals. However, before therapies that target gene expression can be developed the molecular mechanisms that underlie the generation of these sensory cells must first be understood. The long-term goal of this study is to uncover the molecular mechanisms regulating the initiation of hair cell differentiation in the developing mammalian cochlea. Our lab has found the heterochronic Lin28b/let-7 axis to be expressed during cochlea differentiation, overlapping with the onset of hair cell differentiation. Studies in other models organisms including C. elegans and mouse embryonic stem cells have uncovered the importance of this axis in regulating developmental progression and differentiation. In this study we will use well-established genetic tools, includin mouse transgenics and retroviral-mediated gene delivery, to disrupt the expression of the Lin28b/let-7 axis during cochlea sensory epithelium differentiation. Using molecular and cellular biological techniques we will determine the effects of disrupting this pathway on hair cell differentiation and maturation. It is our hope that the experiments put forth in this proposal will advance our understanding of the genetic mechanisms underlying the development and maintenance of sensory cells in the cochlea.